Electro explosive device having a surface spark gap



Feb. 16, 1965 A. NO BLE 6 ELECTRO EXPLQSIVE DEVICE HAVING A SURFACE SPARK GAP Filed Aug. 14, 1963 2 Sheets-Sheet 1 23 L5 I2 I4 INVENTOR LOWELL A. NOBLE ATTORNEY L. A. NOBLE Feb. 16, 1965 ELECTRO EXPLOSIVE DEVICE HAVING A SURFACE SPARK GAP 2 Sheets-Sheet 2 Filed Aug. 14, 1963 INVENTOR LOWELL A. NOBLE Mix/8% ATTORNEY United States Patent Office 3,169,482 Patented Feb. 16, 1965 3,169,482 ELECTRO EXPLOSIVE DEVICE HAVING A SURFACE SPARK 'GAP Lowell A. Noble, Hiilsborough, Calif, assignor to Eitel- McCullough, Inc., San Carlos, Calif., a corporation of California Filed Aug. 14, 1963, Ser. No. 392,176 15Claims. (Cl. 10228) This invention relates to electro explosive devices, such as detonators and squibs, and more particularly to a surface spark gap detonator or squib.

Detonators and squibs are devices which are utilized to ignite or detonate explosive charges. One type of detonator utilizes a thin, gold alloy, bridge wire that is positioned adjacent a relatively small quantity of explosive material. By passing a high current through the bridge wire, the bridge wire vaporizes rapidly (explodes) and detonates the small quantity of explosive material. The detonation of this explosive material is generally utilized to detonate a larger quantity of explosive material. In order to prevent the detonator from being accidentally set off by stray static charges, transient currents or induced radio frequency currents, a spark gap is serially connected with the bridge wire. The gap is designed so that the magnitude of voltage necessary to produce an arc across the gap is larger than the magnitude of stray and transient voltages present in the environment in which the detonator is to be utilized.

Heretofore in the prior art, the spark gap was obtained by precisely spacing two metallic elements to form an air gap. Since the magnitude of the arc voltage is determined by the dimensions of the air gap, it is necessary to closely control the spacing of the two metallic elements because a variation of as little as .001 inch in the air gap spacing can produce a 20% variation in the arc voltage. Accordingly, due to the precise spacing necessary, prior art spark gap detonators are relatively expensive to fabricate. Also, a large number of rejects occur in fabricating prior art spark gap detonators because the spark gap spacing may change during the processes of fabrication, thereby making the finished detonator unit unfit for its intended use. Further, prior art devices sometimes shattered when fired, showering particles that damaged equipment located nearby.

Accordingly, an object of this invention is to provide an improved spark gap detonator.

Another object of this invention is to provide a surface spark gap detonator.

Still another object of this invention is to provide a spark gap detonator that is reliable and readily fabricated.

A further object of this invention is to provide a mechanically rugged spark gap detonator.

These and other objects and advantages of the present invention are obtained in a detonator device which utilizes an elongated nonconductive rod having a conductive coating thereon. A surface spark gap intermediate the ends of the rod is defined by an annular discontinuity in the conductive coating. An insulating body is disposed around the coated rod and a conductive shell has at least a portion of its length surrounding the insulating body. A bridge wire is electrically coupled between a portion of the conductive coating on the rod and the conductive shell. Means are provided for retaining a quantity of explosive charge adjacent to the bridge wire. The are voltage of the spark gap is made independent of ambient pressure and temperature by hermetically confining a quantity of inert. gas, such as nitrogen, around the spark gap.

This invention as well as other objects, features and advantagesthereof will be readily apparent from consideration of the following detailed description relating to the accompanying drawings in which like reference characters designate like or corresponding parts throughout the several views and wherein:

FIGURE 1 illustrates in cross-section a surface spark gap assembly according to the present invention;

FIGURE l-A illustrates in enlarged schematic form a ceramic to metal seal utilized in the assembly of FIG- URE 1;

FIGURE 2 illustrates in cross-section a ceramic body assembly which incorporates the spark gap assembly of FIGURE 1;

FIGURE 3 illustrates in cross-section a complete detonator device in accordance with the present invention;

FIGURE 3-A illustrates an entire end view of the device of FIGURE 3; and

FIGURE 4 is a perspective view of the detonator illus trated in FIGURES 3 and 3-A.

Referring now to FIGURE 1, there is illustrated in cross-section a surface spark gap assembly which includes an elongated cylindrical nonconductive rod 11 having a conductive coating 12 thereon. A spark gap 13 is defined by an annular discontinuity in the conductive coating 12 intermediate the ends of the rod 11. A metallic support cup 14 is secured to one end of the rod 11 and a metalic support tube 15 is secured to the opposite end of the rod.

The rod 11 may be fabricated from any nonconductive material. However, a suitable ceramic, such as beryllia, or a high alumina (more than alumina) ceramic is preferred. Substantially the entire length of the ceramic rod 11 is initially coated with a conductive metallized surface. Although many types of conductive surfaces may be utilized, a suitable surface is formed by mixing powdered manganese and titanium with a powdered metal selected from the group consisting of molybdenum, and tungsten. In order to reduce the work function of the metallized surface, a quantity of thoria may be added to the mix. The metals can be used in the powdered mixture in various forms. For example, the titanium, manganese and molybdenum or tungsten can each be used in the elemental form or in one of their oxide forms. The titanium can also be used in its hydride form. In preparing a preferred powdered mix 0% to 25% by weight of titanium, 40% to 10% by weight of manganese, about 2% by weight of thoria and a major portion by weight of metal selected from the group consisting of molybdenum and tungsten is intimately mixed, such as by ball milling, in a suitable carrier, such as acetone, methyl amyl acetate or mixtures thereof, preferably with a suitable binding agent, such as nitro cellulose. The resulting metallizi'ng paint composition is brushed, sprayed, printed or otherwise applied to the ceramic rod 11. The ceramic rod is then fired in an atmosphere furnace. A reducing atmosphere is desirable and commercial hydrogen has been found to'be satisfactory for this purpse. The temperature in the furnace should be at least as high as the sintering temperature of the metallizing mixture and below the softening temperature of the ceramic. For example, excellent results have been obtained by firing the coated ceramicv at a temperature of about 1425 C. for about 30 minutes.

The spark gap 13 on the ceramic rod 11 is obtained by removing a portion of the metallized surface 12 at an area intermediate the ends of the rod thereby forming an annular discontinuity in the metallized surface. This is accomplished by any suitable means, such as sand blasting or cutting with a wire and diamond paste. Inasmuch as the sintering of the metallized surface 12 on the ceramic rod causes the metallizing materials used to penetrate into the ceramic (a condition generally called .blushing), the blushed portion of the ceramic in the area of the discontinuity or spark gap is also removed.

between the two ends of the metallized ceramic rod 11.

As the method used for forming the annular discontinuity removes the blushed ceramic, the resistance of the ceramic rod 11 will continue to increase until theblushed ceramic is totally absent from the area of discontinuity at which time the resistance of the ceramic rod is substantially infinite. The area of blushed ceramic so removed is exaggerated in FIGURES 1, 2, and 3 for purposes of illustration.

The width of the discontinuity defining the spark gap 13 is determined by the magnitude of voltage desired to cause an arc across the gap 13. In fabricating a detonator device having an arc voltage from 500 to 1000 volts D.C., the width of the spark gap was mils. Since the magnitude of the arc voltage is proportional to the width. of the gap 13, it is necessary to carefully control thelwidth of the spark gap. Once the spark gap 13 is. obtained, the arc voltage of the gap may be determined before continuing with the assembly of a complete detonator.

' In order to aid in securing the end portions of the metallized ceramic rod 11 to the support cup 14 and the support tube 15, a layer of metal is applied to the end portions of the metallized ceramic rod 11. This metal layer may be copper or nickel and is obtained by electro plating or any other suitable means.

The support cup 14 is made from any suitable metal, such as Kovar, and has its open end slotted thereby forming a plurality of circumferentially spaced portions 16 which surround one end of the ceramic rod 11 and are adjacent to the metal layer which covers the end portions of the metallized surface 12. The support cup 14 is brazed to the coated ceramic rod with any suitable brazing material, such as copper. t I

The support tube 15, which is made from any suitable metal, such as Kovar, also has one of its ends slotted to form a plurality of circumferentially spaced portions 17 that engage the other end of the metallized ceramic rod 11 as illustrated in FIGURE 1. The support tube 15 is brazed to the ceramic rod in a manner substantially identical to that of the support cup 14. An enlarged schematic representation of this brazed joint is illustrated in FIG- URE l-A. Reference to FIGURE 1 A shows that a layer of metal 29 covers the metallized surface 12 which is sintered on the ceramic rod 11. Interposed between the metal layer 2%) and the support tube 15 is a layer of copper brazing material 21. Brazing is accomplished in the metal layer 26 provides a much superior brazed joint.

which adheres more tightly to the support tube 15 and the support cup 14.

The interior of the support tube 15 may have a tapered portion 22 which is connected by a longitudinal opening,

23 to the end of the support tube remote from the ceramic rod 11. The reasons for utilizing thisstructure will be discussed hereinbelow in detail.

Referring now to FIGURE 2, there is illustrated a cross-sectional view of a ceramic body assembly that includes a centrally apertured 25 nonconductive body 24 that is disposed around the surface spark gap assembly of FIGURE 1. The centrally apertured cylindrical body 24 is formed from a suitable nonconductive dielectric material, such as beryllia, or a high alumina ceramic. It 13 to be understood, of course, that other insulating materials could be used in place of a ceramic. The outside I circumferential surface of the ceramic body is covered with a conductive or metallized surface and a layer of metal, substantially identical to those discussed hereinsuitable means.

33 is preferably at atmospheric. pressure'and makes the above in detail in conjunction with FIGURES 1 and l'A, which are indicated by the reference character 26. Each end of the aperture 25 also contains a similar conductive coating and metal layer, indicated by the reference characters 2'7 and '28, respectively, which permit hermetically uniting the support cup 14 and the support tube 15 to the inner peripheral surface of the aperture 2.5 at opposite ends I thereof. A copper-silver or copper-tin brazing alloy may. be used to obtain the hermetic seal. The brazing is performedafter the surface sparkagay assembly is inserted into the aperture 25 in the ceramic block 24 such that the support cup 14 lies within and adjacent one end of the ceramic body 24 and the support tube 15 has 'at least a portion of its length extending into the aperture 25 in a manner as illustrated in FIGURE 2. It is believed that the copper vapor produced by the brazing operation, or the blushing of the ceramic by the metallizing paint, produces a semiconductor layer on the surface of the spark gap 13 which decreases the, arc voltage of the spark gap. I

A metallic end cap 31 which may be formed" from anysuitable conductive material, such as nickel, is brazed to the end of the coated ceramic body 24 containing the support cup 14 with a suitable brazing material, such as a copper-silver alloy. Electrically coupled between the support cup 14 and the end cap 31 is a thin bridge wire 32 which is fabricated from any suitable material, such as a gold alloy. The bridge wire may also be formed by nretallizing the ceramic 24 or by'electroplating or vapor depositing nickel or other materials 'on the end surface. Photoetching techniques may also be used to deposit a metal bridge wire across the end of the ceramic 24. In accordance with one embodiment of this invention, the bridge wire selected had a diameter of .0025 inch and had its opposite ends spot welded to the end cap 31'and the support cup 14, respectively. Although FIGURE 2 illustrates the bridge wire as being directly connected between the end cap' 31 and the support cup 14, it is to be understood that the bridge wire may spiral out, or otherwise emanate, from the support cup '14 and terminate on the end cap 31 so that a predetermined length of bridge wire lies adjacent a quantity of explosive charge.

As illustrated in FIGURE 2, the ceramic body surrounds the spark gap 13 in such a manner as to form a tubular area, indicated by the reference character 33, which may be filled with a suitable inert gas, such as nitrogen. This is accomplished by baking out the assembly and evacuating the area 33 by way of the passage comprising the opening 23 in the end of the support tube 15 remote from the spark gap 13, the tapered portion 22 and the slotted space between the circumferentially spaced portions 17. An inert gas, such as nitrogen or sulfur hexafiuoride is then inserted therethrough and is sealed in by pinching off the end of the support tube remote from the spark gap 13, sealing the opening 23 with a suitable material, such as asolder 34, or by any other The inert gas contained within the area arc voltage of the spark gap 13 independent'of varying temperature and pressure conditions.

' Referring to FIGURES, which is a cross-se' tional view of a complete detonator unit taken along the line 3-3 and metal layered surface 26 of the ceramic body 24 is secured to the interior surface of the shell 35 by any suitable means, such as by brazing with a copper-silver or copperstin brazing alloy.

Disposed adjacent and within one end of the elongated hollow cylindrical shell 35 is a quantity of a suitable explosive material 36, such as pentaerythryte tetranitrate The explosive material 36 is contained within the shell 35 by a gasket .37, which may be formed of mica, and a closure plate 38, which may be formed from stainless steel and which is pressure welded to the shell 35. The ceramic body assembly is positioned within the shell 35 such that its end containing the bridge wire 32 lies adjacent the explosive material 36 and its other end lies adjacent the end of the shell 35 remote from the explosive charge.

Reference to FIGURES 3 and 3-A shows that the shell 35 may have a threaded portion 41 along its length and a flanged portion 42 radially extending from the surface of the shell to provide means for threadedly engaging the detonator device of FIGURE 3 in a threaded aperture contained by a wall, tank or any other threaded apertured structure. A plurality of protrusions 43 extend from the wall of the shell 35 adjacent the end'remote from the explosive charge 36 to enable a coaxial connection or coupler to be secured to this end of the detonator unit is illustrated in FIGURE 4.

The opertaion of the device illustrated in FIGURES 3 and 3A is such that a voltage is a plied to the detonator by way of a coaxial line (not shown) which is engaged with the end of the detonator remote from the explosive charge so that the outer conductor of the coaxial line is electrically connected to the shell 35 and the center conductor of the coaxial line is electrically connected to the support tube 15. Whenever the voltage between the support tube (which acts as a center conductor) and the shell (which acts as an outer conductor) exceeds a predetermined amount, a current flows from the support tube 15, through the metallized coating 12, produces an arc across the spark gap 13 causing current to flow through the metallized surface 12 on the other side of the spark gap 13, through the support cup 14, through the bridge wire 32, through the end cap 31, through the shell 35 and back to the coaxial input. The current flow through the bridge wire 32 causes it to rapidly vaporize (explode) which detonates the explosive charge 36. The detonation of the explosive charge 36 is generally utilized to detonate a larger quantity of explosive charge (not shown). The operation of the device is such that the metallized surface 12 of the ceramic rod 11, the support tube 15 and the support cup 14 function as a center conductor and the conductive coating 26 on the exterior of the ceramic body 2 and the shell 35 function as an outer conductor.

The detonation of the explosive charge 36 creates a very high pressure in the neighborhood of 4 million pounds per square inch. The ceramic metal detonator structure illustrated in FIGURE 3 is such that it remains essentially intact under such high pressures even though it was discovered that the force of the exposion drove the metallized ceramic rod 11 into the support tube 15. However, the support tube 15 retained its position and prevented the metallized ceramic rod 11 from escaping the detonator device. If desired, the retaining action of the support tube 15 on the metallized ceramic rod 11 can be increased by tapering the interior surface 22. of the support tube 15 in a manner as illustrated in FIGURES 1, 2 and 3.

Due to the coaxial structure of the detonator device, the outer conductor portion (the shell 35) of the device may be at ground potential thereby shielding the spark gap assembly from radio frequency fields that tend to induce current in the detonator circuit and accidentally set off the detonator. Also the ceramic insulating body 24 interposed between the shell 35 and the spark gap assembly permits the application of a large magnitude of voltage to the device without danger of a voltage break down occurring between the shell 35 and the spark gap assembly.

As will be apparent to those skilled in the art, the fabrication of the device of FIGURE 3 is such that the arc voltage of the spark gap 13 can be checked at various steps in the detonator assembly, and if found defective, easily replaced before completing an entire assembly. Also, once the spark gap 13 is formed on a single piece of ceramic, as discussed hereinabove in detail, and found adequate, there is no reason why the arc voltage .of the gap will change during assembly processes because the gap space remains constant throughout the assembly of the detonator.

It is to be understood that the present invention is not limited to the shell 35 functioning as an electrical conductor; for a conducting rod or conduct-or (not shown) may also pass through the insulating body'24 such that the second rod is insulated from the shell 35 and the spark gap assembly. For this type of construction, the bridge wire 32 would be coupled between the support cup 14 and the second conducting rod. Also, the voltage used to operate this type of detonator structure would be applied by any well known means between the support tube 15 and the end of the second conducting rod remote from the bridge wire 32.

What has been described is a reliable and easily fabricated detonator device which comprises a metallized ceramic rod having a surface spark gap thereon, a conductive shell disposed around said metallized ceramic rod, a ceramic body insulating interposed between the metallized ceramic rod and the conductive shell, and a bridge wire electrically coupled between the metallized ceramic rod and the shell and being positioned adjacent a quantity of explosive material.

It should be understood, of course, that the foregoing disclosure relates only to preferred embodiments of the present invention and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purposes of disclosure, which do not constitute departures from the spirit and scope of the invention set forth in the appended claims.

What is claimed is:

l. A detonator device comprising: a metallized ceramic rod having a surface spark gap intermediate the ends thereof, a conductive shell coaxially disposed around said spark gap on said metallized ceramic rod, a ceramic body coaxially interposed between said metallized ceramic rod and said conductive shell, and a bridge wire electrically coupled between an end portion of said metallized ceramic rod and said conductive shell.

2. The combination according to claim 1 wherein said shell is adapted to contain a quantity of an explosive charge adjacent said bridge wire.

3. A detonator device comprising: at least one elongated nonconductive rod having a conductive coating thereon, a spark gap associated with said coated rod including a discontinuity in the conductive coating intermediate the ends of said rod, an insulating body coaxially disposed around said coated rod, electrical conducting means, and a bridge wire electrically coupled between said electrical conductive means and a portion of said conductive coating on said rod.

4. The combination defined in claim 3 wherein said metallized coating on said ceramic rod comprises a mixture of metal powders including 0% to 25% by weight of titanium, 40% to 10% by weight of manganese and a major proportion by weight of a metal selected from the group consisting of molybdenum and tungsten.

5. A detonator device comprising: .at least one rod having a metallized surface thereon, a spark gap on the surface of said met-allized rod including an annular discontinuity in the metallized surface intermediate the ends of said rod, an insulating body coaxially disposed around said metallized rod, means for confining an inert gas in the vicinity of said spark gap, electrical conducting means associated with said insulating body, and a bridge wire electrically coupled between said conductive means and the portion of said metallized coating located adjacent one side of said spark gap.

6. The combination according to claim 5 further including means to contain a quantity of an explosive material adjacent said bridge wire.

l 7. A detonator device comprising: a cylindrical ceramic body having a metallized surface thereon, a spark gap on said metallized ceramic body including an annular discontinuity in the metallized surface at an area intermediate the ends of said metallized ceramic body, a first conductive section electrically connected to said metallized surface adjacent one end of said metallized ceramic body, a second conductive section electrically connected to said metallized surface adjacent the'other end of said metallized ceramic body, an insulating body disposed around said metallized ceramic body and'having opposite end portions hermetically united to said first and second conductive sections respectively, a quantity of inert gas confined around said spark gap by said first and second conductive sections and said insulating ceramic body, a conductive shell surrounding said insulating ceramic body, and a bridge wire electrically coupled between one of said first and second conductive sections and-said conductive shell.

8. The combination according to claim 7 wherein said.

metallized ceramic body has an annular groove in the surface thereof at the area of discontinuity in the metallized surface defining said spark gap. 7

9. In a device which utilizes a protective spark gap electrically in series with a bridge wire that sets off an explosive charge when sufficient current is passed through the bridge wire, an improved spark gap structure comprising a nonconductive rod having a conductive coating thereon, and a surface spark gap on said coated rod including a discontinuity in the conductive coating intermediate the ends of said rod.

10. The combination defined in claim 9 wherein said rod has an annular groove in the surface thereof at the area of discontinuity'in the conductive coating defining said spark gap.

11. The combination defined in claim 9 wherein said conductive coating comprises a mixture of metal powders including to by weight of titanium, 40% to 10% by weight of manganese and a major proportion by weight of a metal selected from the group consisting of molybdenum and tungsten.

12. A detonator device comprising: a hollow metal body, said metal body functioning as the outer conductor of a coaxial circuit and adapted to receive an explosive charge within and adjacent one end of the metal body, a

centrally apertured first ceramic body located within said metal body with one of its ends adjacent the area adapted to receive an explosive charge and its other end adjacent the other end of said metal body, said first ceramic body ceramic body an'd between said first and second center conductor sections, said second ceramic body having a conductive coating thereon and its opposite ends secured to said first and second center conductor sections, a spark gap including a discontinuity in the conductive coating on 7 said second ceramic body at an area intermediate the ends of second ceramic body, said first and second center conductor sections and said first ceramic body confining a quantity of inert gas around said second ceramic body and saidtspark' gap, and a bridge wire connected between said metal body and said second ce nter conductor section and havingat least a portion of itslength adjacent said explosive charge.

.13. A- detonator device comprising: a hollow metal cylindrical body, said cylindrical body functioning as the outer conductor of a' coaxial circuit, an explosive charge located within and adjacent one end of the cylindrical body, acentrally apertured first cylindrical ceramic body located wtiliin said metal cylindrical body with one of its ends adjacent said explosive charge and its other end adjacent the other end of said metal cylindrical body, said first ceramic body having its'exterior sur ace rnetallized and'brazed to the interior surface of said metal cylinder, a 1 first metal center conductor section having a portion of its length extending into said aperture in said first ceramic .said first and second center conductor sections respectively,

a'spark gap including an annular discontinuity in the metallized surface on said second ceramic body at an area intermediate the ends of second ceramic body, said first and second center conductor sections and said first ceramic body confining aquantity of inert gas around said second;

ceramic body and said spark gap, and a bridge wire electrically connected between said metal body and said second center conductor section and having at least a portion of its length adjacent said explosive charge.

14. The combination according to claim 13 wherein said inert gas is nitrogen.

- 15. The combination according to claim 13 wherein said inert gas is sulfur hexafiuoride.

References Cited in the file of this patent ururno STATES PATENTS 2,986,090 Doerpinghau's May 30, 1961 7 3,106,161 Wallbaum et al. Oct. 8, i963 FOREIGN PATENTS 7 France Mar. 28, 1960 

1. A DETONATOR DEVICE COMPRISING: A METALLIZED CERAMIC ROD HAVING A SURFACE SPARK GAP INTERMEDIATE THE ENDS THEREOF, A CONDUCTIVE SHELL COAXIALLY DISPOSED AROUND SAID SPARK GAP ON SAID METALLIZED CERAMIC ROD, A CERAMIC BODY COAXIALLY INTERPOSED BETWEEN SAID METALLIZED CERAMIC ROD AND SAID CONDUCTIBE SHELL, AND A BRIDGE WIRE ELECTRICALLY COUPLED BETWEEN AN END PORTION OF SAID METALLIZED CERAMIC ROD AND SAID CONDUCTIVE SHELL. 